Temperature tracking of bias supplies for linear RF power amplifiers is essential for stable operation. As is known, all bipolar transistors--including RF power devices--exhibit a negative temperature coefficient in the base-emitter turn-on voltage drop. Consequently, thermal run-away will result when an RF power transistor is biased with a fixed, non-temperature-tracking voltage source.
It is accepted practice to avoid this run-away situation by using a thermally-variable device such as a diode or a diode-connected transistor mounted on the heat sink near the RF power device. This is not ideal, however, because of the thermal lag between the RF device and the sensor due to the thermal resistances and masses involved.
As is known, an improvement in response will result if the temperature-sensing diode is physically as close as possible to the transistor die contained in the RF package. In many practical cases, however, unless a special package is employed in which the sensing diode is internal and is mounted next to the transistor die, this implies mounting the sensing diode on the RF device package. A problem arises with this approach, however, since the high RF field around the RF power transistor causes rectification in the temperature-sensing diode with a polarity opposite to the bias voltage. As a result, the bias voltage decreases with increasing RF drive and the amplifier input-output transfer characteristic displays a significant non-linearity. In some severe cases, a low-frequency blocking type oscillation may also result.
As a result of the above, there is a need for an improved RF power amplifier.